KINETIC-ANALYSIS OF 2 SIMULTANEOUSLY ACTIVATED K-MEDIA L( CURRENTS INROOT CELL PROTOPLASTS OF PLANTAGO)

Two different, simultaneously activated outward rectifying K+ currents were analyzed in the plasmalemma of root cortex protoplasts of Planta go media. Their gating is dependent on the diffusion potential for K(E(K)). The threshold potential was more negative than E(K) allowing s mall inward currents at potentials below E(K) thereby keeping cells wi th little pump activity in the K state (Vogelzang & Prins, 1994). Time and voltage dependence of the outward rectifying K+ currents have bee n analyzed with Hodgkin-Huxley-like (HH) models. Dynamic responses of whole cell currents to pulse potentials were analyzed with two voltage dependent functions, the Boltzmann distribution for open probability per gate and the transition rate towards the open state (alpha). The t ransition rate in the opposite direction (beta), was calculated from a lpha and the Boltzmann distribution. These functions were used for an integral analysis of activation and deactivation currents measured ove r a range of pulse potentials. Both whole cell and single channel data were used for the determination of the number of closed and open stat es. The effects of single channel flickering on time response and ampl itude of tail currents were added to the model. The dominant K+ channe l present in the plasmalemma of P. media has a characteristic nonlinea r single channel I-V curve reducing the amplitude of whole cell curren ts at positive potentials. To compensate for this nonlinearity, a four state translocator model was added to the whole cell open probability model. The analysis presented here provides a general basis for the s tudy and comparison of K+ channel kinetics in plant protoplasts.